[Java] Avoid casting ByteBuffer to Buffer (#6785)

* Revert "avoiding even more NoSuchMethod exceptions (#6729)"

This reverts commit 6fb2c90d9e.

* Revert "avoiding more NoSuchMethod exceptions (#6671)"

This reverts commit 752c7b576d.

* Revert "avoiding NoSuchMethod exception (#6658)"

This reverts commit 813d3632ec.

* Use Java 8 for Kotlin Linux builds to verify

java/com/google/flatbuffers/Utf8Old.java

@@ -17,7 +17,6 @@
 package com.google.flatbuffers;
 
 import java.nio.ByteBuffer;
-import java.nio.Buffer;
 import java.nio.CharBuffer;
 import java.nio.charset.CharacterCodingException;
 import java.nio.charset.CharsetDecoder;
@@ -56,7 +55,7 @@ public class Utf8Old extends Utf8 {
     if (cache.lastOutput == null || cache.lastOutput.capacity() < estimated) {
       cache.lastOutput = ByteBuffer.allocate(Math.max(128, estimated));
     }
-    ((Buffer) cache.lastOutput).clear();
+    cache.lastOutput.clear();
     cache.lastInput = in;
     CharBuffer wrap = (in instanceof CharBuffer) ?
                           (CharBuffer) in : CharBuffer.wrap(in);
@@ -68,7 +67,7 @@ public class Utf8Old extends Utf8 {
         throw new IllegalArgumentException("bad character encoding", e);
       }
     }
-    ((Buffer) cache.lastOutput).flip();
+    cache.lastOutput.flip();
     return cache.lastOutput.remaining();
   }
 
@@ -87,11 +86,11 @@ public class Utf8Old extends Utf8 {
   public String decodeUtf8(ByteBuffer buffer, int offset, int length) {
     CharsetDecoder decoder = CACHE.get().decoder;
     decoder.reset();
-    Buffer b = ((Buffer) buffer).duplicate();
-    b.position(offset);
-    b.limit(offset + length);
+    buffer = buffer.duplicate();
+    buffer.position(offset);
+    buffer.limit(offset + length);
     try {
-      CharBuffer result = decoder.decode((ByteBuffer) b);
+      CharBuffer result = decoder.decode(buffer);
       return result.toString();
     } catch (CharacterCodingException e) {
       throw new IllegalArgumentException("Bad encoding", e);